1. Technical Field
The present invention relates to a chromatic dispersion compensation device, and more particularly to a chromatic dispersion compensation device including an array of micromirrors to selectively delay portions of an optical channel of a wavelength division multiplexed (WDM) optical signal.
2. Description of Related Art
Fiber optic networks provide high speed, high capacity communication that can exceed 20 gigabytes per second. The transmission data comprises a series of light pulses propagating along an optical fiber. Each light pulse is composed of different spectral components that propagate through the optical fibers at different speeds with higher wavelength components traveling slower than the lower wavelength components of the light pulses in non-dispersion compensated fibers, due to the variation of refractive index of the fiber core. This effect, known as chromatic dispersion, results in the spreading out or broadening of the light pulses.
Chromatic dispersion becomes increasingly pronounced at higher bit rates, such as rates greater than 2.5 gigabytes per second. As the transmission rates increase, the light pulses become closer and closer. At these higher bit rates, the broadening of the light pulses (or bits) may result in the overlapping of adjacent bits, and thus reduces the sensitivity of the receiver to distinguish the bits. Consequently, chromatic dispersion is a limiting factor to the faster transmission of data.
Some known dispersion compensation devices include dispersion compensation fibers (DCF) and chirped grating disposed in an optical fiber.
A dispersion compensation fiber has an index variation with λ being opposite in sign to that of a typical optical fiber transmission fiber within the optical network. For example, the optical fibers of a network typically have a positive index profile. A relatively long compensating fiber having a negative index profile is disposed in-line with the optical fiber. The summation of the two opposite fibers cancels the chromatic dispersion of the network.
A chirped fiber Bragg grating is a special fiber with spatially modulated refractive index that is designed so that longer (shorter) wavelength components are reflected at a farther distance along the chirped fiber Bragg grating than are the shorter (longer) wavelength components. A chirped fiber Bragg grating of this sort is coupled to a fiber communications system through an optical circulator. By causing certain wavelength components to travel longer distances than other wavelength components, a controlled delay is added to those components and opposite dispersion can be added to a pulse. Unfortunately, a chirped fiber Bragg grating has a very narrow bandwidth for reflecting pulses, and therefore cannot provide a wavelength band sufficient to compensate for light including many wavelengths, such as a wavelength division multiplexed light. A number of chirped fiber Bragg gratings may be cascaded for wavelength multiplexed signals, but this results in an expensive system.
A related technology is that of strain-tuned fiber gratings. It is known that the resonant wavelength of an individual fiber grating may be tuned by either tensile strain (i.e., stretching) or compressive strain. Strain tuning has been applied to a uniform grating used for filtering and to a chirped grating used for dispersion compensation.
Present dispersion compensation methods, as described above, have several shortfalls. Dispersion compensators formed by long lengths of compensating fiber normally have a higher loss than conventional fiber. They are also cumbersome and their properties can only be changed in discrete steps since change is accomplished by switching lengths of fiber in and out of the compensator. A chirped fiber Bragg grating has a narrow bandwidth, and even if strain tuned, is only adjustable over a small range. Additionally, a chirped grating typically requires a length on the order of meters for full compensation. What is needed is a way to provide a reliable, fully adjustable (tunable), broadband dispersion compensator with a wide dynamic range. Additionally, such a compensator could be enhanced through the development of a monitoring and control system that monitors dispersion asynchronously and controls dispersion compensating elements using relatively inexpensive hardware.